1. Technical Field
The present invention relates to semiconductor photolithography methods and, more particularly, to antireflection coatings for use with photolithography.
2. Description of the Related Art
One step in the fabrication of semiconductor devices such as integrated circuits is the formation of a substrate pattern over a semiconductor wafer surface through photolithographic masking and etching. A photoresist coating over a substrate is selectively exposed to activating radiation directed through a mask defining the desired conductor pattern. After photoresist development, the photoresist layer constitutes a relief image mask over the substrate. The relief image mask defines open areas over the substrate in a desired image pattern to be transferred to the substrate. The image is transferred to the surface of the substrate by surface modification of the substrate in a negative image of the pattern within the photoresist coating, such as by removal of a portion of the substrate by an etching process or by implantation of an atomic species into the substrate. The etching is often done in a plasma etch reactor in which a plasma of ions reacts with and etches away the exposed substrate. During these processes, the coating of the photoresist in the image pattern functions as a protective mask to prevent surface modification of the substrate underlying the photoresist mask. The resolution of the image transferred to the substrate is dependent upon the resolution within the imaged photoresist coating.
There are factors in addition to the resolution capability of the photoresist used that influence the quality or resolution of the image transferred to a photoresist masked substrate. For example, with reflective integrated circuit substrates, such as aluminum, exposure of a photoresist coating causes reflection of diffused activating radiation (light) from the integrated circuit substrate back into the photoresist coating. Standard photoresists are susceptible to surface reflections which degrade the fine-line images required for integrated circuit manufacture. This degradation occurs due to reflection of diffused light from the integrated circuit substrate back into the photoresist layer resulting in exposure of the photoresist layer in areas where imaging is not desired. Another common result of surface reflections is the formation of xe2x80x9cnotchesxe2x80x9d in conductive lines in certain regions because of unwanted exposure of photoresist by reflected light. These xe2x80x9cnotchesxe2x80x9d can cause the device to fail, or even worse, to be unreliable.
To prevent reflection of activating radiation into a photoresist coating, it is well known to provide antireflective layers (ARC""s) between a substrate and a photoresist layer. These antireflective layers typically comprise an absorbing dye dispersed in a polymer binder though some polymers contain sufficient chromophores whereby a dye is not required. When used, the dye is selected to absorb and attenuate radiation at the wavelength used to expose the photoresist layer thus reducing the incidence of radiation reflected back into the photoresist layer. During the conventional processing of an integrated circuit substrate coated with the combination of an antireflective layer and a photoresist layer, the photoresist is exposed to activating radiation and developed to form a relief image, i.e., portions of the photoresist layer are removed by development with a liquid developer and portions remain as a mask defining a desired pattern. To alter the underlying substrate, the antireflective layer must be removed to bare the substrate in a desired image. Removal of the antireflective layer may be by dissolution with a liquid that simultaneously dissolves both the photoresist and the antireflective layer or by dry etching such as with an oxygen plasma.
Unfortunately, present antireflective coatings are less than 100 percent effective and are often difficult to remove. Furthermore, removal of the antireflective coating often results in degradation of important device properties and inconsistent performance of antireflection coatings due to thickness variation and other factors limits performance of photolithography. Therefore, it is desirable to provide an antireflective coating and a photolithographic process that results in up to 100 percent efficiency and that can be more easily removed from the underlying substrate.
The present invention provides a method of fabricating an integrated circuit using photolithography and an antireflective coating. According to a preferred embodiment, an antireflective coating is formed on a substrate wherein the antireflective coating is electrically polarizable. A photoresist coating is formed on the antireflective coating on a side opposite from the substrate and the photoresist is exposed to activating radiation. The antireflective coating is subjected to an applied electric field at substantially the same time as the photoresist is exposed to activating radiation. The radiation absorption coefficient of said antireflective coating is increased and the refractive index of said antireflective coating is changed to be substantially equal to the refractive index of said photoresist coating.